284 APPENDIX. 



To Prepare Flies or Mosquitoes for Transmission Through the Mails. 



Wrap the insect carefully in a piece of tissue-paper (toilet-paper answers). Impreg- 

 nate sawdust with 5% carbolic acid solution and fill around the folded insects in the 

 box containing them. (Barely moisten.) 



C PREPARATION OF NORMAL SOLUTIONS. 



A normal solution is one which contains the hydrogen equivalent of an element, 

 expressed in grams, dissolved in sufficient distilled water to make 1000 c.c. The 

 hydrogen equivalent is the atomic weight of any element divided by its valence. 

 In a base, salt or acid we use the molecular weight in grams divided by valence. 



What may be considered as the valence of a base is shown by the number of 

 hydroxyls combined with it; that of an acid by the number of replaceable hydrogen 

 atoms which it contains. 



To make a normal solution, dissolve in distilled water a weight in grams equal 

 to the sum of the atomic weights of the substance, divided by its valence, and make 

 up the volume to exactly 1000 c.c. 



NaOH is univalent. Na = 23. O = 16. H = i. Dissolve 40 grams NaOH 

 in water and make up to exactly 1000 c.c. 



Oxalic acid is COOH COOH + 2 H 2 O which gives it a molecular weight of 

 126. As it contains two carboxyl groups it is dibasic, and it is necessary to divide 

 the molecular weight by 2, so that for a normal solution of oxalic acid we dissolve 

 63 grams in a volume of distilled water made up to 1000 c.c. 



If a chemical laboratory is not accessible one may prepare normal solutions with 

 an error so slight as to be unimportant in clinical work in the following way: 



Sodium hydrate being very hygroscopic, it is impossible to accurately prepare a 

 normal solution by directly weighing out the substance. Instead, select perfect 

 crystals of oxalic acid, such as can be obtained in a drug store, and weigh out on 

 the most accurate apothecary scales obtainable exactly 6.3 grams of the most per- 

 fect crystals in the bottle. Put these preferably in a volumetric flask and make 

 up with distilled water to 1000 c.c. Less accurate is the use of a measuring cylinder. 

 If care is used this should give N/io solution of oxalic acid in which the error is less 

 than i%. 



Having N/io acid at hand, we may prepare N/io NaOH in the following way: 

 Weigh out an excess of sodium hydrate (5 grams of stick caustic soda) and dissolve 

 in 1 100 c.c. of distilled water. Take up xoc.c. of this solution with a pipette and 

 let it run into a beaker. Add six drops of phenolphthalein solution. This gives a 

 violet-pink color. Fill the burette with the N/io oxalic-acid solution and let it run 

 into the sodium-hydrate solution in the beaker until the pink is just discharged. 

 Reading off the number of c.c. of the N/io acid used, we know the strength of the 

 sodium-hydrate solution. It is well to repeat the titration and take an average. 



If 10.5 c.c. of the oxalic-acid solution were required it would show that the 

 sodium-hydrate solution was stronger than N/io, as only 10 c.c. would have been 



